Circuit breakers are special switches or protective gear and/or switchgear that is/are designed such that it/they can, under many fault conditions, such as ground fault, phase opposition, overload and short circuit, for example, in an electrical network, such as e.g. a low voltage network, switch on and off, hold and safely disconnect load currents, high overload currents and short-circuit currents. Such switchgear is used as incoming-feeder, distribution, tie and outgoing-feeder circuit-breakers in electrical systems. Such switches are also used for the switching and protection of motors, capacitors, generators, transformers, bus bars and cables.
Circuit breakers can be used particularly in low voltage networks, i.e. networks having voltages or rated voltages of up to 1000 volts AC or 1500 volts DC. These circuit breakers have sensor units, such as current sensors, that measure the electric current flowing through the switch.
Particularly in the low voltage range, circuit breakers are used for current intensities or rated current ranges from 16 to 6300 amps, particularly in the range from 16 to 1600 amps, more specifically in the range from 63 or 125 to 1600 amps, for example as a compact circuit breaker, and also from 630 to 6300 amps, more specifically from 1200 to 6300 amps, for example as an open or air circuit breaker.
Usually, circuit breakers have a tripping unit to interrupt the electrical circuit, said tripping unit being provided with contacts, for example, that open or close the electrical circuit. In addition, a circuit breaker has a control unit that may be implemented centrally or locally. When current limit values and/or current/time period limit values, which are defined by a characteristic curve, for example, are exceeded, the circuit breaker interrupts the electrical circuit.
In this case, a first current limit value Ii, which needs to be exceeded for a first, only short, period of time t1 for interruption purposes, is the current value for what is known as instantaneous protection or short-circuit protection, for example. Instantaneous means interruption with the minimum possible or a set short tripping time for the circuit breaker.
A second, for example adjustable, current limit value Isd for the circuit breaker, which needs to be present for a second period of time t2 for interruption purposes, is the current limit value for what is known as short-term-delayed interruption or tripping, for example. This second current limit value Isd defines the level of the current from which short-term-delayed interruption begins. In this case, the current needs to be there for the second period of time t2 before interruption takes place. The short-term-delayed interruption or tripping can have an adjustable second period of time t2 or delay.
When the current rises above the second current limit value Isd, the second period of time t2 can decrease with increasing current in order to prompt interruption.
When the current rises above the second current limit value Isd for a time greater than the set delay, for example, interruption takes place after the second period of time t2.
When the current rises above the second current limit value Isd, it is possible, given a current-dependent characteristic of the short-term-delayed tripping, for the second period of time t2 to decrease with increasing current in order to prompt interruption.
When the first current limit value Ii is reached, instantaneous interruption of the electrical circuit frequently takes place.
Circuit breakers are used for distributing electric power. When a fault occurs in the power distribution system, the circuit breaker arranged immediately upstream of the fault is intended to recognize the fault current, to trip and to disconnect the current. This is referred to as selectivity or selective tripping.
When multiple circuit breakers are connected in series, for example between energy source and energy sink, or current source and load, all circuit breakers situated in the path frequently recognize an overload or short circuit. Disconnection by the circuit breaker closest to the fault under many conditions is desirable.